This invention relates to a method of producing a thin layer of semiconductor material. The thin layer produced can possibly be provided with electronic components.
The invention permits the production of thin layers of either monocrystalline or polycrystalline or even amorphous semiconductor and, for example the production of substrates of the Silicon on Insulator type or the production of self-supporting thin layers of monocrystalline semiconductor. Electronic circuits and/or microstructures can be either completely or in part created in these layers or in these substrates.
It is known that implanting ions of a rare gas or of hydrogen in a semiconductor material induces the formation of microcavities at a depth proximate to the mean penetration depth of the ions. The document FR-A-2 681 472 discloses a method which uses this property in order to obtain a thin film of semiconductor. This method consists of subjecting a wafer of the desired semiconductor material that includes a flat face, to the following steps:
a first implantation step by bombarding the flat face of the wafer with ions creating, within the volume of the wafer and at a depth proximate to the penetration depth of the ions, a layer of microcavities separating the wafer into a lower region constituting the mass of the substrate and an upper region constituting the thin film, the ions being chosen from among the ions of rare gases or of hydrogen gas and the temperature of the wafer being maintained below the temperature at which the implanted ions can escape from the semiconductor by diffusion;
a second step of bringing the flat face of the wafer into close contact with a support made up of at least one layer of rigid material. This close contact may be created, for example using an adhesive substance, or by the effect of a preliminary preparation of the surfaces and possibly a thermal and/or electrostatic treatment in order to promote interatomic bonding between the support and the wafer;
a third step of thermal treatment of the wafer-support assembly at a temperature greater than the temperature at which the implantation was carried out and sufficient to create, through a crystal rearrangement effect in the wafer and through the pressure of the microcavities, a separation between the thin film and the mass of the substrate. This temperature is, for example 500xc2x0 C. for silicon.
This implantation is capable of creating a layer of gaseous microbubbles. This layer of microbubbles thus created within the volume of the wafer; at a depth proximate to the mean penetration depth of the ions demarcates, within the volume of the wafer, two regions separated by this layer one region intended to constitute the thin film and one region forming the rest of the substrate.
According to the implantation conditions, after implantation of a gas, such as, for example hydrogen, cavities or microbubbles may or may not be observable by transmission electronic microscopy. In the case of silicon, it can be obtained microcavities, the size of which can vary from a few nm to several hundreds of nm. Hence, particularly when the implantation temperature is low, these cavities are only observable during the thermal treatment stage, a step during which nucleation is brought about in order to end up with the coalescence of the microcavities at the end of the thermal treatment.
The method described in the document FR-A-2 681 472 does not allow the production of electronic circuits in or at the surface of the flat face of the wafer after the ion implantation step. Indeed, the creation of such circuits implies the carrying out of certain classic micro-electronics operations (diffusion annealing, deposition etc.) that require thermal treatment stages (typically from 400xc2x0 C. to 700xc2x0 C.) according to the steps for silicon. At these temperatures, blisters form on the surface of the flat face of the implanted wafer. By way of example, for an implantation of hydrogen ions at a dose of 5.1016 protons/cm2 and at 100 keV energy in a silicon wafer, a thermal treatment carried out at 500xc2x0 C. for 30 min. leads to degradation of 50% of the surface of the flat face of the wafer, this degradation resulting in the appearance of blisters and to their bursting. It is then no longer possible to properly ensure that the flat face of the wafer is brought into close contact with the support (which will be called the applicator in the subsequent description) so as to detach the semiconductor layer from the rest of the wafer.
This phenomenon of the formation of blisters and craters in the surface of a silicon wafer implanted with hydrogen ions after annealing has been discussed in the article xe2x80x9cInvestigation of the bubble formation mechanism in a-Si:H films by Fourier-transform infrared microspectroscopyxe2x80x9d by Y. Mishima and T. Yagishita, that appeared in the J. Appl. Phys. 64 (8), Oct. 15, 1988, pages 3972-3974.
This invention has been conceived in order to improve the method described in the document FR-A-2 681 472. After a step of ion implantation within a range or appropriate doses and before the separation step, it allows to carry out a thermal treatment of the part of the wafer corresponding to the future thin layer, in particular between 400xc2x0 C. and 700xc2x0 C. for silicon, without degrading the surface condition of the flat face of the wafer and without separation of the thin layer. This intermediate thermal treatment can form part of the operations for developing electronic components or can be applied for other reasons.
The invention is also applicable in the case where the thickness of the thin layer is sufficient to confer good mechanical characteristics on it, in which case it is not necessary to use an applicator in order to achieve the separation of the thin layer from the rest of the wafer, but where it is desired, despite everything, to avoid surface defects in the flat face.
Therefore an objective of the invention is a method of production of a thin layer of semiconductor material from a wafer of said material having a flat face, including an ion implantation step consisting of bombarding said flat face with ions chosen from among the ions of rare gases or of hydrogen, at a specific temperature and a specific dose in order to create, in a plane called a reference plane and situated at a depth proximate to the mean depth of penetration of the ions, microcavities, the method also including a subsequent thermal treatment step at a temperature sufficient to achieve separation of the wafer into two parts, across the reference plane, the part situated on the side of the flat face constituting the thin layer, characterised in that:
the ion implantation step is carried out with an ion dose between a minimum dose and a maximum dose, the minimum dose being that from which there will be sufficient creation of microcavities to obtain the embrittlement of the wafer along the reference plane, the maximum dose, or critical dose being that above which, during the thermal treatment step, there is separation of the wafer,
a separation step of separating the wafer into two parts, across the reference plane, is provided after or during the thermal treatment step, this separation step comprising the application of mechanical forces between the two parts of the wafer.
These mechanical forces can be tensile forces, shear forces or bending forces applied alone or in combination.
In the application, by microcavities, one understands cavities that can be of any form; for example, the cavities can be of a flat shape, that is to say of small height (a few interatomic distances) or of substantially spherical shape or any other different shape. These cavities can contain a free gaseous phase and/or atoms of gas arising from the implanted ions fixed to atoms of the material forming the walls of the cavities. In Anglo-Saxon terminology, these cavities are generally called xe2x80x9cplateletsxe2x80x9d, xe2x80x9cmicroblistersxe2x80x9d or even xe2x80x9cbubblesxe2x80x9d.
The thermal treatment carried out with the purpose of achieving separation of the thin layer from the rest of the wafer, allows the microcavities to be brought to a stable state. Indeed, under the effect of temperature, the microcavities coalesce to reach a final definitive condition. Hence, the temperature is chosen in such a way that this condition is obtained.
According to document FR-A-2 681 472, the doses implanted are such that, under the effect of the thermal treatment, a layer of microcavities is obtained that allows the separation to be achieved directly.
According to this invention, the doses implanted are insufficient to achieve a separation during the thermal treatment, the doses implanted only allow an embrittlement of the wafer at the reference plane, the separation requires an extra step of applying mechanical forces. Furthermore, the critical dose, as defined in the invention, is less than the dose at which during the ion implantation and thermal treatment steps, there is blister formation on the flat face of the wafer. The problem of blisters does not therefore arise in the invention.
The method according to the invention can include, between the thermal treatment step and the separation step, a step consisting of producing all or part of at least one electronic component in the part of the wafer before forming the thin layer.
If the production of this electronic component requires phases of heat treatment, these are preferably carried out at a temperature below that of the thermal treatment.
If needed, just before the separation step, an extra step is provided, consisting of bringing said wafer, on the side of said flat face, into close contact with and rigidly fixing it to a support through which mechanical forces such as tensile and/or shearing forces will be applied.
This support can be a flexible support, for example a sheet of Kapton(copyright). It can also be a rigid support such as a wafer of oxidised silicon.